There is illustrated in FIG. 7 a structure of a light-emitting device formed of a conventional AlGaInP-base semiconductor material. That is, as shown in the figure, an n-type GaAs substrate 11 has thereon a light-emitting layer forming portion 19. The light-emitting layer forming portion 19 comprises an n-type lower clad layer 12 formed of an n-type AlGaInP-base semiconductor material on the substrate 11, an active layer 13 formed of a non-doped AlGaInP-base semiconductor material on the lower clad layer 12, and an upper clad layer 14 formed of a p-type AlGaInP-base semiconductor material on the active layer 13. Furthermore, a window layer 15 is formed on the surface of the light-emitting layer forming portion 19. These layers are sequentially formed on the substrate 11 by epitaxial growth. An upper electrode 17 is formed of gold or aluminum through a GaAs contact layer 16, while a lower electrode 18 is formed also gold or aluminum on a back surface of the substrate 11.
The light-emitting device of this kind is of a double hetero structure, in which the mixed crystal ratio for the light-emitting layer forming portion 19, i.e., the clad layers 12, 14 and the active layer 13, is determined so as to confine carriers within the active layer 13 sandwiched between the both clad layers 12, 14 for enhancing efficiency of emission of light. Since the light-emitting device of this type is adapted to radiate light through a top-surface side, the upper electrode 17 is provided in such a minimal size for supplying electric current that the light is less obstructed or absorbed by the upper electrode.
However, in the conventional light-emitting device formed of the AlGaInP-base semiconductor material as above, the light created by the light-emitting layer forming portion 19 having the active layer 13 sandwiched by the clad layers 12, 14 travels, besides upwards, sideways and downward of the chip to be radiated outward. However, the semiconductor substrate 11 of GaAs is of not transparent for the light thus emitted so that the light traveling toward the semiconductor substrate is absorbed by the substrate. To this end, the light radiation available is limited only to the portion of the light emitted upward, without being obstructed by the upper electrode 17, from the light-emitting layer forming portion 19. Therefore, there is a problem that the efficiency of light radiation is low, and accordingly the brightness of light is also low.
On the other hand, there can be thought of using the material such as an AlGaAs-base material which is epitaxially grown over a light-emitting layer forming portion 19 for a substrate 11. The AlGaAs-base material has a composition with a mixture crystal ratio of Al greater than 0.7 so that the longer wavelength portion of visible light than that of yellow light is less absorbed by the material. However, the AlGaAs-base material is liable to oxidize so that it is practically impossible to grow an AlGaInP-base material on the AlGaAs-base material.